I was trying my electrical tester the other day, and discovered that I had managed to forget to turn it off again.

So, I decided to hunt for rechargeable batteries.

Alkaline batteries: 1.5V cells, 9V made from 6 cells, or about 9V.NiMH, 1.25V cells, One could make 9V from 7 cells, or about 8.75V.NiCad 1.25V cells, One could make 9V from 7 cells, or about 8.75V.Lithium Ion, 3.6V cells (or so),Most of the 9V Lithium batteries I'm seeing are about 7.2V to 7.8V (2 cells). I would have probably done 3 cells, or about 10.8V.

I was trying my electrical tester the other day, and discovered that I had managed to forget to turn it off again.

So, I decided to hunt for rechargeable batteries.

Alkaline batteries: 1.5V cells, 9V made from 6 cells, or about 9V.NiMH, 1.25V cells, One could make 9V from 7 cells, or about 8.75V.NiCad 1.25V cells, One could make 9V from 7 cells, or about 8.75V.Lithium Ion, 3.6V cells (or so),Most of the 9V Lithium batteries I'm seeing are about 7.2V to 7.8V (2 cells). I would have probably done 3 cells, or about 10.8V.

Anyway, how important are the voltages? A little high? A little low?

If your meter requires a voltage supply of 9 volts, then I would advise that most types of "9 volt" batteries are nominally 9 volts, but for most of their life, they supply ~80% nominal, which is 7.2 volts (and 1.5 volt batteries supply 1.2 volts, etc).

So, is this what you say you are seeing in the Lithium batteries — that so-called "9 volt" batteries are supplying only 7.2 to 7.8 volts? If so, then you will more-or-less see then same in other batteries. For example, your alkaline 1.5 volt cells will supply about 1.2 volts for most of their life, and a 6-cell configuration will supply only 7.2 volts most of its life.

However, I see the bottom line not the voltage supplied but a solution to your complaint — forgetting to turn off the meter. This is solved by the ability of the battery type to withstand deep discharges, or more likely, full discharges. I'm not a battery expert, but I've read that NiCads apparently can take a full discharge.

Hopefully someone else can speak more knowledgeably on which battery types best survive deep/full discharges.

You may also want to consider other criteria, such as how "green" the different battery types are.

Correct,Apparently some battery types have problems with deep discharges, including Lead Acid, and Lithium Ion.

Many lithium ion systems have some kind of protection to shut them off during deep discharge. However, it depends on whether this protection is in fact in the battery, or in the device, or a combination of the two.

Some devices such as calculators are designed to turn themselves off after an hour or two, but apparently that doesn't include my multitester.

Keep in mind, when comparing 9V Lithium to 9V Alkaline. The fully charged Lithium might be around 7.5V.However, the minimal charge is about 2.5V per cell, or around 5V for a 9V battery.

With an Alkaline battery (1.5V cells), the discharge curves show they are mostly discharged at around 1V cells. So, 6 cells, gives one 6V for fully discharged. So, in a device like a smoke detector that has a low voltage sensor, it may deal well with the Lithium batteries, perhaps cutting off a little early, but still giving a significant amount of usage.

So, perhaps that is the answer.While devices might be able to handle the 10.8V, 3 cell lithium batteries, by using the lower range, 5V to 7.8V, that puts the cells mostly in the lower end of the alkaline battery range of 6V to 9V, and may even be able to take advantage of low voltage detection systems in some devices. Running it at 7.5V to 10.8V, the low voltage protection would not detect the discharged cells.

And, assuming the same overall battery size, the watts from a 2 cell or 3 cell battery would be similar.

One of my electrical testers uses 3 x CR2032 batteries at 3V each. I'm going to test it with 3xLIR2032 batteries at 3.6V each (rechargeable), although I suppose I could try it with just two cells.

Alkaline batteries, depending on brand, typically begin life at about 1.55V with no load. The voltage instantaneously drops while you're pulling a heavy load from them owing to "internal resistance". An alkaline battery can generally be regarded as being spent/exhausted when its open circuit or light-load voltage has dropped to about 0.9 to 1.0V per cell. An appliance intended for use with alkaline batteries should be designed to function down to ~70% of its stated/nominal supply voltage.

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"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"

Does your tester not have some kind of "auto-off" function you can enable?

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"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"